CATV services are normally provided by transmitting a plurality of television (TV) programs from a head end of the system over television channels of a transmission line to a plurality of subscribers. It is known that such TV programs can be transmitted first to each of a plurality of distribution locations and from each distribution location over a plurality of subscriber lines to the subscribers. A converter at each subscriber location permits the subscriber to select whichever non-premium television channel the subscriber may want.
The provider may also transmit from the head end a plurality of jammed or scrambled TV programs over a plurality of premium TV channels. Those subscribers which, for an added fee, are permitted to receive premium channels, are provided with converters containing de-scramblers for permitting premium programs to be coherently received. In addition, pay-per-view (PPV) is a service sometimes provided to enable paying subscribers to watch a scheduled event at a particular time. This service may require, for example, that the subscriber communicate by telephone to the provider at the service office a request that he be permitted access to the scheduled event, whereupon the service provider remotely actuates a de-scrambler at the subscriber's location for permitting him to receive the scheduled event on a specified TV channel for a specified fee.
It has been recognized that these methods for providing premium service inherently suffer from several problems. First, the converters containing de-scramblers at each subscriber location are somewhat "user unfriendly"; i.e., such apparatus is sometimes difficult for the subscriber to operate. The converters are subject to tampering because they are located on subscriber premises. Another problem of a jamming system with jamming signals generated from the head end is the need for cascaded trap filters along the video signal path at each subscriber location. This results in degradation of the signal-to-noise ratio of many of the video channels. Also, transmission of the jamming signals tends to distort the premium TV signals.
Various techniques have been proposed for applying jamming signals at the distribution locations. This reduces the distortion of the premium signal by the jamming signals because the distance from the distribution locations to the subscriber locations is normally much smaller than the distance from the head end to the distribution locations, and there is typically a smaller number of active components.
Proposals have been made to add scrambling signals only to those TV channels of subscriber lines corresponding to premium signals which the individual subscribers are not entitled to receive. While such systems would eliminate all propagation of scrambling signals on premium video channels subscribers are entitled to receive, and would thereby eliminate the need for de-scramblers at the subscriber locations, they are difficult to operate reliably and they impose additional costs.
Thus, there has been a long-felt need for methods for delivering premium CATV services to selected subscribers, which methods are reliable, relatively inexpensive and easy to install and operate.